Drug loading and release properties of ion-exchange resin complexes as a drug delivery matrix

Complexes of ion-exchange resins and dextromethorphan (DM), a model drug, were prepared using different methods including a batch and a column method with different functional groups, ion-exchange capacity, degree of crosslinking, and resin particle size. Drug loading efficiency, release profiles, and scanning electron micrographs were also investigated. Most of the functional groups of resins were loaded with DM after the completion of a double batch method and it was recommended for drug loading into the ion-exchange resin. Using a column method, drug loading could be monitored by simply measuring changes in the pH of the reaction medium since as complex formation reached completion, the pH returned to the initial pH of the eluent due to the limited amount of functional groups available for the exchange. DM could be loaded up to the ratio of 3 (drug): 1 (resin), depending on the physicochemical properties of the resin. As the crosslinking ratio and particle size increased, the drug loading and release rate decreased due to the reduced effective diffusion coefficient and surface area. Assuming that the resin particles are uniform spheres of radius r, release mechanism was evaluated using plots of a Bt − t relationship, where B (= π2Di/r2) and t are the rate constant and time, respectively. Di represents the effective diffusion coefficient of DM inside the resin. The Bt − t plots displayed a straight line indicating that the diffusion of DM in the resin matrix is the rate-controlling step.